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The human brain, once thought to lose much of its flexibility after childhood, continues remodeling itself throughout life—recovering from injuries, learning new skills, and adapting to challenges—a remarkable capacity for change called neuroplasticity.

Songbirds show similar neuroplasticity on a seasonal basis, with changes in their brain enabling elaborate songs during breeding seasons, and to recover full vocal repertoires even after years of reduced singing. This raises a fundamental question: How does the brain preserve these complex abilities when they are not being actively used?

A new study from the Max Planck Institute for Biological Intelligence tackles this by focusing on female canaries, which do not normally sing, but nevertheless contain the full neural machinery for song. Like humans have specialized brain regions for language, songbirds evolved these specialized regions for song—and singing ability in canaries is partly inherited and partly learned through practice.

By giving females testosterone—a hormone that rises in males during breeding season—the team could watch what happens over several weeks as this dormant singing ability switches on and the females fine-tuned their newly found voices.

The work appears in Proceedings of the National Academy of Sciences.

Ramping up activity

Using traditional imaging methods, scientists had previously observed the HVC—a key brain region for song—appearing much larger in the images of singing as compared to non-singing canaries, e.g. during breeding season.

However, by tracking individual brain cells in the HVC, a combination of advanced microscopy and studies of gene expression revealed something different. Neurons didn't move apart or multiply as it might have seemed—instead, they were becoming more active, strengthening connections, ramping up activity, and shifting gene expression, and easier to detect, creating an illusionary growth in the anatomical images.

"Rather than spacing out to expand the brain region, gene mapping showed that testosterone orchestrates these changes throughout the HVC without changing its size," says Shouwen Ma, researcher of the Department of Behavioural Neurobiology at the Max Planck Institute for Biological Intelligence in Seewiesen.

"Crucially, this means that the HVC maintains its size and neural architecture even when not in use, allowing birds to regain complex singing abilities even after years without singing. What is so exciting for me about this discovery is that it's a great illustration of how the brain doesn't need to rebuild structures from scratch: The neural machinery stays ready, just waiting to be switched on and fine-tuned."

Persistent capacity for singing

The capacity for song proved remarkably persistent—the researchers even induced singing in seven-year-old females, well beyond their typical lifespan in the wild.

"This project has been a significant and cumulative effort for the department. We have refined genomic tools to assess the molecular basis of singing behavior of canaries at high resolution. This ranges from genome sequencing to sophisticated bioinformatic methods that visualize gene expression to reveal brain architecture. This diversification of knowledge lays the foundation for new research areas, such as understanding how species cope with different hormonal systems," says Carolina Frankl-Vilches, a researcher in the Department of Behavioural Neurobiology.

"By studying how individual cells change their behavior, we're uncovering fundamental principles of brain plasticity and how brains retain capacity for innovative changes throughout life," says Manfred Gahr, head of the department.

"Understanding these mechanisms—how hormones trigger changes in brain cells, how neural circuits preserve their structure and flexibility—could shed light on the broader principles in brain plasticity and therefore broader questions such as how aging brains maintain their capacity to adjust and better understand the mechanisms to promote recovery after stroke or injury."